Aircraft are typically controlled by moveable aerodynamic surfaces on the wing and tail for controlling the altitude and attitude of the aircraft in flight, including roll, pitch, and yaw. Small aircraft can use simple mechanical linkages to transmit the operator's control inputs to these surfaces. Larger and faster aircraft need power assisted or fully-powered control systems. Most powered control systems use mechanical linkages to command the surface actuators, but “fly-by-wire” and “fly-by-light” are becoming more common. In these systems, the mechanical linkage is replaced by a computerized command and feedback loop.
While manually operated control systems provide a direct and reliable link between the operator's input and the movement of the aircraft control surfaces, these mechanical systems tend to have a poor failure tolerance, and as such, require redundant and independent load paths for continued safe flight and landing, in the event of a failure. As a result, additional hardware and mechanisms are required to provide this redundancy.
With a manual system, the work available at the control surface is limited by the force and travel that an operator can apply to the cockpit controls. As the speed and size of aircraft increase, manual control of the aircraft becomes inadequate to effectively maneuver the aircraft. Various means, including servo tabs and fully powered systems, have been employed to provide the additional force necessary to move the control surfaces. However, these systems add complexity and additional failure modes. For example, fully powered systems are dependent on their power sources, which themselves must be highly redundant. Failure of a fully powered system, without having a manual reversion or back-up, would result in complete failure, and lack of control, of any aircraft control surface. Furthermore, fully powered systems do not provide natural force feedback to the operator. Therefore, artificial feel systems must be added when such a control system is utilized.
The evolution of aircraft design and technology have resulted in a class of aircraft, such as many larger business class jets, that cannot be completely controlled manually and require power-aided controls and would benefit from computer-aided controls. However, fully powered and completely computerized controls, such as fly-by-wire, are not warranted.